Many different processes have been employed to cure (vulcanize) green tire carcasses in a mold, but most favored are machines of the Bag-O-Matic type made by McNeil Corp., and Auto-Form made by NRM Corp., in which machines a high internal bladder pressure is used while limiting the vulcanizing temperature (see for example, U.S. Pat. Nos. 2,066,265; 3,489,833; 3,579,626 and 4,027,543; and "The Applied Science of Rubber", W. J. S. Naunton (1961), pp 1053-1083). The latter publication describes various tire curing systems which have been used including those employing circulating hot water, dead-end hot water, and steam. Some use high-pressure steam in the bladder followed by lower pressure steam; some use high-pressure steam followed by high-pressure dead-end hot water; and others use high pressure steam followed by circulating high-pressure hot water. During molding, the pressurized heated bladder is forced against the inner surface of the tire which has been precoated with an inside tire paint, until vulcanization is completed. The bladder is then depressurized, deflated and withdrawn from the interior of the tire.
Tire paints are so termed because a thin film of a liquid dispersion is painted onto a green tire. The tire paint is dried on the green tire which is then vulcanized (cured) in a tire mold. Without tire paints a vulcanized tire could not be removed from a mold without the tire being damaged. Therefore, the tire paint must be so adapted as to satisfy critical surface requirements of a tire being molded, during all stages of the molding cycle, yet function as a release agent which also facilitates the finishing of the tire. A tire's outer and inner surfaces are each contacted during the molding cycle by the surface of the mold and the bladder respectively, and the dynamics of movement of the outer tire surface and the inner tire surface are not the same. Therefore, it is conventional to use an outside tire paint for the outer surface, and an inside tire paint for the inner surface; and, the outside and inside tire paints generally have different ingredients.
It would be more convenient and far more economical to use outside and inside tire paints in which the ingredients are substantially the same and only their relative amounts in the paints differs. A composition for such paints and methods for producing each are disclosed in our U.S. Pat. Nos. 4,325,852 and 4,329,265 the disclosures of which are incorporated by reference thereto as if fully set forth herein.
Many commercial inside and outside tire paints used in the past have been hydrocarbon solvent based ("solvent based"), typically having gasoline as the solvent. When solvent based paint is applied to the green tire, evaporation of the solvent leads to pollution of the atmosphere, besides being a waste of a valuable natural resource. Solvent based tire paints have been made obsolete in this country because silicone release agents ("silicone") now in use may be used in aqueous systems. Silicone is especially useful to overcome the high initial static friction when the tire is slid into the mold. Silicone provides the requisite slip well, but it also can lead to tire defects. Splice areas where tire components are spliced together are especially vulnerable to seepage of the silicone between spliced components, for example, the tread splice and the inner splice. If either of these splice areas have the slightest opening, silicone will seep into the opening and prevent self-adherence of the rubber during vulcanization. U.S. Pat. No. 3,507,247 discloses a typical prior art solvent based tire paint together with an apparatus for applying said paint. It would be desirable to have a tire paint for both the inside and outside, requiring only a change in the ratio of ingredients in the recipe, which paint is not only free of hydrocarbon solvent and silicone, but also free of rubber latex ("latex-free").
The outside green tire paint serves (i) to permit surface rubber to slip as it comes in contact with the metal mold; (ii) as a release agent when at the end of the vulcanization cycle, the outside tire surface must be separated from the mold; (iii) to allow air which is trapped between the outside tire surface and the mold to escape (bleed); and (iv) to enhance the appearance of the finished tire, thus providing a cosmetic function.
One of the principal functions of the inside green paint is to act as a lubricant between the tire inner liner and the curing bladder both during the loading or shaping stage and the stripping stage of the molding operation. Lubricity is particularly needed during inflation of the bladder in the initial and shaping stages because there is substantial relative movement between the contacting surfaces of the bladder and the tire inner liner. Unless there is adequate lubrication provided between the tire and inner liner, there is a tendency for the bladder to buckle, which may result in misshaping of the tire and consequent rejection of the tire. At the end of the molding operation when the bladder is collapsed and the tire is stripped from the bladder, there is again considerable relative movement between the contacting surfaces of the bladder and the now cured tire inner liner. Unless adequate lubrication is provided between the bladder and inner liner, the bladder tends to stick to the cured tire. This causes excessive wear and roughening of the bladder, which results in reduced bladder life. When the bladder sticks to the cured tire, it also may cause a delay in the molding operation.
Another of the principal functions of the inside green tire paint is to get rid of unwanted air bubbles between the tire inner liner and the bladder at the beginning of the shaping operation and to promote entry of air between the bladder and the tire inner liner at the end of the molding operation to avoid adhesion of the tire inner liner to the bladder when the bladder is evacuated prior to withdrawal from within the tire. The entrapment of a substantial amount of air in the form of bubbles between the bladder and the inner liner, and, failure of the liner to separate from the bladder upon evacuation of the bladder at the end of the molding cycle, may each lead to such severe defects in the molded tire as to require it to be rejected.
The paints utilized in our '852 and '265 patents consist essentially of the same, or chemically analogous materials and avoid the use of hydrocarbon solvents and silicone release agents. Though hydrocarbon solvents have generally been discontinued in the tire industry for environmental reasons, silicone release agents are still popular and economical if a person is inclined to overlook the number of defective tires (percent rejection) attributable to the use of the silicone.
A scrutiny of the listed ingredients of each recipe in our aforementioned patents discloses that the essential difference between the inside and outside tire paints is that (i) the inside tire paint includes a minor amount of carnauba wax relative to the amount of paraffin wax, and (ii) the outside tire paint contains much more rubber latex than the inside tire paint (which is termed a low rubber latex paint), and no carnauba wax.
Our early experimental work with a low rubber latex inside tire paint showed that when it was used as an outside tire paint, the outside surface of the cured tire flaked. We concluded that the rubber latex acts as a binder which keeps the paint on the tire, and that it was essential to limit the percentage of defective tires.
In further tests with the low rubber latex inside tire paint having the carnauba wax content of Example 1 (about 6% by weight of the paint) discovered that with certain large truck tires requiring long cure times, there was an unexpectedly high percentage of defects which appeared to be attributable to inadequate lubricity between the bladder and the inner surface of the green tire. As will be evident from data presented hereinbelow, the problem was high static friction during the initial phase of curing.
Since the solids of the rubber latex ("latex" for brevity) in the inside tire paint accounted for less than 1% by weight of the paint it seemed unlikely that it adversely affected lubricity, but because natural rubber latex is inherently tacky, we resolved to remove it from the recipe, and did. We were surprised to find that the lubricity appeared to be improved, and a very low percent of tires with defective inner surfaces confirmed that the rubber latex was best left out of the recipe.
Having tested the latex-free inside tire paint on numerous tires of various configurations it eventually occured to me that the rubber latex in the outside tire paint may not be contributing as much to lubricity as we once believed, though it was conjecture on our part that, lubricity or lack thereof notwithstanding, the latex was probably still essential to avoid deleterious flaking and to enhance the visual appeal of the finished tire. Only actually testing a latex free outside tire paint could satisfy our presentiment that the absence of latex was essential to an effective paint. This, we did. The results are embodied in this invention.